Copper(II) hydroxide, also known as cupric hydroxide and having the chemical formula Cu(OH)2, has a wide variety of commercially important uses, including as a mordant and pigment in dyeing textile and paper fibers, in the preparation of catalysts and other copper compounds, in marine paints, and in fungicides and bactericides.
Even under ambient conditions copper(II) hydroxide is thermodynamically unstable relative to decomposition to copper(II) oxide. This inherent instability complicates the manufacture, distribution and storage of copper(II) hydroxide and compositions containing it. The change of chemical composition of copper(II) hydroxide to copper(II) oxide can be gradual or rapid and is typically accompanied by change of the blue color characteristic of copper(II) hydroxide to a more greenish hue and ultimately the black color of copper(II) oxide.
Although copper(II) hydroxide is thermodynamically unstable, whether or not significant decomposition occurs as well as the rate of decomposition (i.e. the kinetics of decomposition) is affected by a variety of conditions. Increasing temperature is well known to promote the decomposition of copper(II) hydroxide to copper(II) oxide. H. B. Weiser et al. J. Am. Chem. Soc. 1942, 64, 503-508 report that under otherwise constant conditions, the rate of transformation of copper(II) hydroxide to copper(II) oxide is higher the smaller the crystals of the copper(II) hydroxide. H. B. Weiser et al. also report that copper(II) hydroxide gels prepared by addition of a slight excess of alkali to a cupric salt solution decompose even at room temperature and even a trace of alkali accelerates the decomposition.
The patent literature discloses a variety of processes for the commercial manufacture of copper(II) hydroxide. U.S. Pat. Nos. 2,924,505, 3,428,731, 3,628,920 and Re 24,324 disclose processes involving phosphate. U.S. Pat. Nos. 4,490,337 and 4,808,406 disclose processes involving carbonate; the latter process provides a product comprising considerable copper carbonate in addition to copper hydroxide. U.S. Pat. Nos. 1,800,828, 1,867,357, 2,525,242, 2,536,096 and 3,635,668 discloses processes involving ammonia. The processes of U.S. Pat. Nos. 2,525,242 and 2,536,096 involve oxidation of copper metal in the presence of ammonia, and U.S. Pat. No. 4,944,935 discloses a similar process substituting ammonium ion for all or part of the ammonia. The other processes start with a soluble copper salt, typically copper(II) sulfate. U.S. Pat. No. 4,404,169, European Patent EP 80226 B1 and PCT Patent Publication WO 02/083566 A2 describe processes starting with copper(II) oxychloride. J. Komorowska-Kulik, Zeszyty Naukowe Politechniki Sltaskiej, Series: Chemistry 2001, 142, 59-66 discloses a process wherein an aqueous suspension of copper(II) oxychloride is contacted with aqueous sodium hydroxide in the presence of glycerol as stabilizer. None of these processes pertain to stabilizing copper(II) hydroxide prepared by another process.
The processes commercially used to prepare copper(II) hydroxide provide products in forms different from the gels described by Weiser et al. and have greater kinetic stability. However, reaction temperatures for these processes are limited to not much above room temperature, and the storage life of the copper(II) hydroxide products of these processes may be limited, particularly at temperatures significantly above room temperature. U.S. Pat. No. 3,428,731 states that crystalline copper hydroxide prepared by the phosphate-based process of U.S. Pat. No. Re 24,324 is stable for indefinite periods at temperatures not exceeding 120° F. (48.9° C.). U.S. Pat. No. 2,536,096 indicates for an ammonia-based oxidation process that at temperatures above about 87° F. (31° C.) cupric oxide is also formed, and this becomes the product at temperatures above 140° F. (60° C.). U.S. Pat. No. 4,490,337 states that the reaction slurry in the carbonate-based process may decompose at temperatures above 32° C., but discloses examples drying the product at higher temperatures; a product dried at 60° C. was found to be highly amorphous by x-ray analysis.
For the phosphate-based process disclosed in U.S. Pat. Nos. Re 24,324 and 3,428,731 a reaction sequence is suggested where sodium phosphate is used to form copper(II) sodium phosphate as an intermediate, which is then treated with sodium hydroxide to form copper(II) hydroxide and regenerate sodium phosphate. U.S. Pat. No. 3,428,731 states that from the reaction mixture a dry, solid product is obtained by separating the solids from the mother liquor, washing, drying and grinding. U.S. Pat. No. 3,428,731 also states that a minor amount of phosphate can be included in the product as calcium phosphate by substituting calcium hydroxide for at least part of the sodium hydroxide.
U.S. Pat. No. 2,924,505 describes a process for preparing copper hydroxide containing a bound phosphorus content (expressed as PO4) of substantially not less than 3.5%. Any substantial deviation below the minimum average content of bound phosphorus is said to lead to the formation of cupric oxide. A non-crystalline product is claimed. The product is prepared by adding streams of aqueous copper sulfate and sodium hydroxide solutions to a slurry formed from sodium phosphate and copper sulfate. The precipitated product is washed to remove water-soluble impurities, including unbound phosphate, and then dried at a temperature around 140° F. (60° C.).
U.S. Pat. No. 3,628,920 describes a process for preparing a copper hydroxide-phosphorus complex having a bound phosphorus content of at least about 2% by weight calculated as P2O5. The complex is prepared by mixing copper sulfate and phosphoric acid with sodium hydroxide such that the pH is maintained between about 10 to 11.5. As the complex formed is stated to decompose to copper oxide when held at temperatures of 112° F. (44° C.) for even 0.5 to 10 minutes, a reaction temperature range of 85-110° F. (29-43° C.) is specified, and temperatures below 85° F. (29° C.) are recommended for longer reaction times. The product is washed thoroughly and then dried at temperatures up to about 180° F. (82° C.). The product in form of a wet cake is stated to be stable at temperatures up to 150° F. (66° C.) over relatively long periods of time.
U.S. Pat. No. 4,404,169 describes a process for preparing copper(II) hydroxide involving contacting copper oxychloride with an alkali metal or alkaline earth metal hydroxide in the presence of phosphate ions as stabilizer. The temperature is stated to desirably not exceed 35° C., preferably 20 to 25° C., to avoid transformation of some of the copper hydroxide to the oxide. The precipitated copper hydroxide is recovered, washed, and again suspended in an aqueous phase and treated with an acid phosphate to bring the pH to a value of 7.5 to 9. The copper hydroxide is then separated from the aqueous mixture, washed, and suspended in water or dried. European Patent 80226 B1 describes the aqueous mixture being used as a fungicide. Neither reference provides stability data.
PCT Patent Publication WO 02/083566 A2 describes a process for preparing copper hydroxide containing copper phosphate. In this process an aqueous solution of cupric oxychloride is combined with aqueous sodium hydroxide in a continuous reactor comprising a high-shear agitation system. The reaction mass is held at 20-24° C. to complete the reaction, and then treated with an aqueous solution of orthophosphoric acid. In a strongly agitated reactor, the reaction mixture is then brought to pH 8-8.2 by adding an aqueous solution of cupric chloride, resulting in precipitation of copper phosphate. The reaction mass is collected using vacuum filtration, washed with softened water, and dried using a spray drier. No information regarding stability is disclosed.
German Patent Publication DE 19543803A1 describes a process for preparing copper(II) hydroxide phosphate from copper(II) hydroxide and a stoichiometric amount of phosphoric acid for use as an artist's pigment. The product is collected by filtration; the filter cake is washed and then dried.
Minimizing decomposition of copper(II) hydroxide during its storage and use is important for many of its applications including as a fungicide and bactericide. To evaluate the stability of plant protection products, the Food and Agriculture Organization of the United Nations has described an accelerated storage procedure, Method MT 46, involving heating at 54±2° C. for 14 days (see Manual on Development and Use of FAO Specifications for Plant Protection Products, Fifth Edition, January 1999, sections 3.6.2 and 5.1.5). The elevated temperature in the procedure serves to assess stability if the products are stored or used at elevated temperatures, and also to simulate the aging process at ambient conditions but in a shorter period of time. The decomposition of copper(II) hydroxide to copper(II) oxide can be evaluated by a number of methods, including x-ray diffraction and colorimetric measurement.
Although effective methods are available to prepare copper(II) hydroxide products and to assess their kinetic stability, copper(II) hydroxide products having better storage stability and greater resistance to heat are still needed. Particularly desirable are methods for stabilizing (i.e. increasing the kinetic stability) of copper(II) hydroxide products prepared by known processes.